The third derivative
[src]
/ 2 \
| 27 32*x 36*x |
3*|-8 + ----------------- + ------ + ----------------|
| / 2\ 2/x\ 2 / 2\ /x\|
| \9 + x /*atan |-| 9 + x \9 + x /*atan|-||
\ \3/ \3//
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2 _________
/ 2\ / /x\
8*\9 + x / * / atan|-|
\/ \3/
$$\frac{3 \left(\frac{32 x^{2}}{x^{2} + 9} + \frac{36 x}{\left(x^{2} + 9\right) \operatorname{atan}{\left(\frac{x}{3} \right)}} - 8 + \frac{27}{\left(x^{2} + 9\right) \operatorname{atan}^{2}{\left(\frac{x}{3} \right)}}\right)}{8 \left(x^{2} + 9\right)^{2} \sqrt{\operatorname{atan}{\left(\frac{x}{3} \right)}}}$$